I know that extra hours in the morning equates to more people in that park that day and correspondingly slightly less people in the other parks.
Do extra hours in evening effect traffic in the morning at the park that has them?
No, not at all, and you're absolutely right about the morning hours making that park more crowded the whole day, and the other parks less crowded. But evening hours don't have that effect, in my experience
I love evening extra magic hours - because of the lower crowds. The place is absolutely packed at parade and fireworks time - and then it empties out - We are usually there in February - so I'm not too sure what happens the rest of the year - but those Sunday nights in Magic Kingdom are fantastic. You feel like you're there with some private party or something - that is until you get back to Main Street USA and see the hundreds of people still shopping at the Emporium. Even Peter Pan is a short line.
I just don't have the energy to stay up and take advantage of the later hours of evening EMH. But I do like the morning hours, especially in the MK and Epcot.
I know what you mean, when we were there in May, MK didn't close (regular hours) until 11pm - we were too tired to do EMH. However, during the time we usually go in February, MK closes by 9 so EMH starts a lot earlier. We usually go over there around 6 or 7, get a bite to eat and then start doing rides - we'll stay until 11:30. That is actually quite relaxing - My brother and his older kids can go until 1am - they do everything but swim in the moat around Cinderella's castle. I've always been more of your "never say die" type myself - but I'll be 45 this year and find that I wilt a little easier than I used to - but don't tell anyone it's a secret ;-)